package org.bouncycastle.crypto.generators;

import java.math.BigInteger;

import org.bouncycastle.crypto.AsymmetricCipherKeyPair;
import org.bouncycastle.crypto.AsymmetricCipherKeyPairGenerator;
import org.bouncycastle.crypto.KeyGenerationParameters;
import org.bouncycastle.crypto.params.ElGamalKeyGenerationParameters;
import org.bouncycastle.crypto.params.ElGamalParameters;
import org.bouncycastle.crypto.params.ElGamalPrivateKeyParameters;
import org.bouncycastle.crypto.params.ElGamalPublicKeyParameters;

/**
 * a ElGamal key pair generator.
 * <p>
 * This generates keys consistent for use with ElGamal as described in page 164 of "Handbook of Applied Cryptography".
 */
public class ElGamalKeyPairGenerator implements AsymmetricCipherKeyPairGenerator {
    private ElGamalKeyGenerationParameters param;

    public void init(KeyGenerationParameters param) {
        this.param = (ElGamalKeyGenerationParameters) param;
    }

    public AsymmetricCipherKeyPair generateKeyPair() {
        BigInteger p, g, x, y;
        int qLength = param.getStrength() - 1;
        ElGamalParameters elParams = param.getParameters();

        p = elParams.getP();
        g = elParams.getG();

        //
        // calculate the private key
        //
        x = new BigInteger(qLength, param.getRandom());

        //
        // calculate the public key.
        //
        y = g.modPow(x, p);

        return new AsymmetricCipherKeyPair(new ElGamalPublicKeyParameters(y, elParams), new ElGamalPrivateKeyParameters(x, elParams));
    }
}
